Spin coating apparatus having a horizontally linearly movable wafer holder

ABSTRACT

A semiconductor wafer 11 is mounted on an elongated member 18, one end of which is rotatable about a transverse axis (14), thereby to distribute a liquid on the upper surface of the wafer more evenly. In order to stabilize the rotation of the elongated member, a second elongated member is preferably attached end-to-end to the elongated member (18) and rotates with it. A counterweight (26) in the second elongated member moves during the rotation such that the distance between the wafer and the central axis and the distance between the center of the counterweight and the axis are substantially equal. The weight distribution is approximately symmetrical about the axis and the structure is dynamically stabilized. The counterweight and the wafer assembly may be moved during rotation by applying air pressure from a source (23) to pistons (13,26) in the two elongated member.

This is a division of application Ser. No. 396,200 filed Aug. 21, 1989now U.S. Pat. No. 5,032,492, granted Jul. 16, 1991.

TECHNICAL FIELD

This invention relates to methods and apparatus for making integratedcircuits and, more particularly, to methods and apparatus fordistributing a liquid such as a photoresist uniformly on the uppersurface of a semiconductor wafer.

BACKGROUND OF THE INVENTION

Semiconductor integrated circuit device fabrication normally requiresrepeated processes of photolithographic masking and etching on an uppersurface of a wafer of a semiconductor material such as silicon. Each ofthese processes requires spreading a thin layer of a liquid photoresistmaterial on the surface of the wafer, selectively exposing it through amask to actinic light, developing the photoresist to define a maskpattern on the wafer surface and using this mask pattern to describe apermanent pattern on the wafer. The permanent pattern on the wafer mayin turn be used to control selective etching, diffusion, ionimplantation, metal deposition, oxide deposition, and other processesthat describe the finished integrated circuit.

As integrated circuits have become more complex, the requirements foraccuracy and resolution of the photolithographic processing have becomemore stringent. Requirements for line-width definition of less than onemicron are becoming increasingly commonplace. As a consequence, greateraccuracy is required at all stages of the photolithographic processing.

The liquid photoresist is typically dispensed on one surface of thewafer with the wafer being held in a vacuum chuck that is subsequentlyrotated to disperse the liquid over the entire surface. While thismethod is suitable for many purposes, it does tend to result innon-uniformities in thickness of the photoresist that can affect theuniformity and consequent accuracy of the resulting pattern to bedeveloped. Development of the photoresist likewise requires a uniformdistribution of developing fluid for a uniform development of thedesired pattern. Finally, the steps of stripping photoresist, liquidetching, and cleaning of the wafer all require a uniform distribution offluid. The effects of non-uniform distribution of any of these liquidscan be somewhat overcome by dispensing rather large amounts of theliquid at different locations over the surface, but in themass-production of integrated circuits, this solution can be quitewasteful of the liquid and therefore expensive, and in any event, itonly reduces rather eliminates coating thickness non-uniformities.

SUMMARY OF THE INVENTION

During the rotation of the wafer at a sufficiently high rate of speed,distribution of a liquid on its surface is made more uniform by movingthe axis of the rotation of the wafer during such rotation. Therotational axis can be varied so as to be at any location on the waferor at a location outside the surface of the wafer.

In apparatus for practicing the invention, the wafer is mounted on anelongated member, one end of which is rotatable about a transverse axis.A support member for the wafer is mounted on the elongated member suchas to move controllably along the elongated member during its rotation,thereby moving the location of the wafer relative to the axis ofrotation during wafer rotation. In order to stabilize the rotation ofthe elongated member, a second elongated member is preferably attachedend-to-end to the elongated member and rotates with it. A counterweightin the second elongated member moves during the rotation such that thedistance between the wafer and the central axis and the distance betweenthe center of the counterweight and the axis are substantially equal.With the counterweight weighing approximately the same as the wafer andassociated assembly, the weight distribution is approximatelysymmetrical about the axis, and the structure is dynamically stabilized.Both the counterweight and the wafer assembly may be moved duringrotation by applying air pressure to pistons in the two elongatedmembers as will be described more fully below.

These and other objects, features and advantages of the invention willbe better understood from a consideration of the following detaileddescription taken in conjunction with the accompanying drawing in which:

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a schematic view of apparatus for distributing fluid on thesurface of a semiconductor wafer in accordance with an illustrativeembodiment of the invention.

FIG. 2 is a top view of FIG. 1.

FIG. 3 is a view taken along lines 3--3 of FIG. 2.

FIG. 4 is a sectional view of the apparatus of FIG. 1, but with thesemiconductor wafer assembly in a different location.

FIG. 5 is a top view of FIG. 4.

FIG. 6 is a schematic view of another embodiment of the invention.

FIG. 7 is a schematic view of yet another embodiment of the invention.

DETAILED DESCRIPTION

Referring now to FIGS. 1 and 2, there is shown schematically apparatus10 for distributing a liquid on the upper surface of a semiconductorwafer 11 in accordance with an illustrative embodiment of the invention.The wafer 11 is mounted on a wafer holder 12 which is rigidly coupled toa piston 13. Piston 13 and holder 12 may include permanent magnets whichprovide the rigid coupling. For example, a magnet portion 16 of thewafer holder may be a rare earth permanent magnet which is attractedstrongly to a permanent magnet in the piston 13. The entire assembly 10is rotatable about the central axis 14 by a motor schematically shown as15. Any fluid such as photoresist liquid that has been dispensed on theupper surface of the wafer 11 will then be distributed about the uppersurface in the conventional manner.

While such liquid distribution has been successfully used for years inthe semiconductor industry, it can be shown that such distributionnormally results in non-uniformities in the thickness of the distributedliquid coating. This can be appreciated by considering that thetangential velocity v at any point on the wafer is a function ofdistance from the central axis, or,

    v=ωr.sup.2                                           (1)

where ω is the angular velocity of the wafer, and r is the distance ofthe location under consideration from the central axis. It can be seenthat, at the central axis, the tangential velocity is zero and there istherefore no centrifugal force to cause the liquid to be distributed. Asthe distance from the center increases, the tangential velocityincreases, thereby exerting greater centrifugal force on the liquid tobe distributed and causing it to be distributed progressively morethinly.

The purpose of the apparatus of FIG. 1 is to cause the axis of rotationof the wafer to be varied with respect to the wafer during its rotationand thereby make the liquid distribution more uniform. The piston 13 islocated within a cylindrical passage 17 of a first elongated member 18.In the position shown, a spring 19 bears against one side of the pistonand a stop 20 bears against its opposite side. Passage 17 is connectedthrough a channel 22 and a rotatable pressure fitting (not shown) to asource of controllable air pressure 23. Channel 22 and passage 17between piston 13 and channel 22 are air-tight so that, when airpressure is applied from source 23, the piston 13 is forced against thebias of spring 19 along the elongated member 18.

FIG. 4 shows the piston 13 in its extreme position due to theapplication of maximum air pressure. Wafer holder 12 is also located atthe same extreme position because of its coupling with piston 13. Toavoid any compression of air, the left end of member 18 should be open,or at least not be air-tight.

As illustrated in FIG. 3, wafer holder 12 is free to move along theupper surface of elongated member 18 and is guided by a pair of tracks24. Although the absolute location of the axis of rotation 14 does notchange, one can see that in FIG. 4 the location of the axis of rotationrelative to the location of wafer 11 has changed. By moving the piston13 along elongated member 18, one moves the axis of rotation. of thewafer from a location at the center of the wafer as shown in FIG. 1 to alocation outside the surface of the wafer 11 as shown in FIG. 4, (Forpurposes of this document, "rotation" of a body is intended to includemovement about an axis that may be external of the body as well as anaxis extending through the body. Also, movement of one body "relative"to another body is intended to imply that either body may bestationary.) As a consequence of relative movement of the central axiswith respect to the wafer, one changes the angular velocity at variouslocations on the wafer and therefore the centrifugal forces on liquidson the surface of the wafer. These changes tend to make the distributionof the liquid on the surface more uniform. Specifically, they prevent arelative accumulation of liquid at the center of the wafer.

Referring again to FIG. 1, integrated with the elongated member 18 is asecond elongated member 25 containing a piston 26 which is biased by aspring element 27. The central passage 28 of the second elongated membercommunicates with channel 22 and therefore with the controllable sourceof air pressure 23. Piston 26 is designed to have approximately the sameweight as the combined weight of piston 13, holder 12 and wafer 11. As aconsequence, the effect on piston 26 of the application of air pressureis the same as that on piston 13; and as piston 13 is driven by airpressure along member 18, piston 26 is driven an equal and oppositedistance along second elongated member 25.

FIG. 4 shows pistons 13 and 26 after being driven an equal and oppositedistance from center line 14. This has the effect of dynamicallystabilizing the apparatus 10 by making symmetrical the loads on theapparatus during rotation about the central axis 14. As the air pressureis reduced, the springs drive pistons 13 and 26 back toward the centerin precisely the same manner, thus to maintain stability. It should benoted that in FIG. 4 the sole rotation of wafer 11 is around centralaxis 14; i.e., there is no rotation of the wafer around an axis throughits center.

The invention is particularly useful for distributing liquid photoresistalong the top of the semiconductor wafer prior to selective exposurethrough a photomask to actinic light. After such exposure, a developingliquid is dispensed on the upper surface of the wafer and the wafer isagain rotated with the apparatus shown, and with the method describedabove for distributing the developing fluid. The developing fluid reactswith the exposed photoresist so as to describe a pattern in accordancewith the selective exposure of the photoresist to actinic light. Theresulting mask over the surface of the wafer is thereafter used tocontrol etching, diffusion, ion implantation, metal deposition, oxidedeposition, and/or other processes that describe the finished integratedcircuits. Uniform distribution in accordance with the invention givesthe same relative resolution capabilities at all locations on thesurface of the wafer, both during the exposure of the photoresist andduring its development. After development, the invention may be used todistribute other liquids on the wafer surface, e.g., liquids forstripping photoresist, etchant liquids, and cleaning liquids may bedistributed in this manner. In all such cases, the properties of theliquids are optimized and waste of the liquids is minimized for thereasons given above.

Because of the use of different fluids for different purposes, it may bedesired that the wafer be rotated at any of various speeds between about100 rotations per minute (rpm) and about 5000 rpm. At relatively lowerspeeds there should be no problem in operating the apparatus, but ahigher speeds care should be taken that the magnetic attraction betweenpiston 13 and wafer holder 12 is sufficient to overcome the centrifugalforce on holder 12. If, due to the high speed desired for distributionof a particularly viscous liquid, one prefers that a direct mechanicalconnection be made between wafer holder 12 and piston 13, a slot andappropriate gaskets in a slot permitting such direct linkage may beprovided. The gaskets would keep the part of passage 17 between piston13 and channel 22 air-tight as is known in art.

FIG. 6 shows alternatively another method of mechanically directlyattaching the piston 13 with the wafer holder 12. The reference numbersof FIG. 6 refer to elements that correspond to those having the samereference numbers of FIG. 1. Since the extreme left end of elongatedmember 18 may be open, one may easily include a rigid U-shaped couplingelement 30 interconnecting piston 13 and wafer holder 12. As piston 13is moved to the left, wafer holder 12 is likewise moved to the left aswas described before; however, for the sake of brevity, separatedrawings showing pistons 13 and 26 in the extreme positions,corresponding to FIG. 4, have not been shown. A U-shaped member which ispart of the counterweight may be attached to piston 26 for greatermechanical symmetry if so desired.

Referring to FIG. 7 there is shown still another embodiment of theinvention in which the first elongated element 18 has been axiallydisplaced with respect to the second elongated element 25. The purposeof this arrangement is to permit pistons 13 and 26 to be axially alignedwhen there is no displacement of the wafer with respect to the centralaxis 14. This obviates the need for offsetting the major portion of thewafer holder 12 from magnet 16 as is required in the embodiment of FIG.1 to permit alignment of the wafer with the central axis. In the designof FIG. 7, it is important that both central passages 17 and 28communicate with channel 22 so as to permit equal forces on pistons 13and 26 during application of the controllable air pressure.

It can be appreciated from the foregoing that a number of mechanicalstructures can be devised for moving the wafer 11 during the course ofrotation. For example, instead of using air pressure to move thepistons, mechanically or electrically driven elements could be used. Ihave found that a counterweight structure is normally desirable toreduce mechanical wear of apparatus 10 during operation, although inprinciple, such counterweight structure is not required. If the wafer ismounted on a rotable disk that is much heavier than the wafer, the wafercould be moved radially along the disk during rotation without anycompensating counterbalance. Various other embodiments and modificationsmay be devised by those skilled in the art without departing from thespirit and scope of the invention.

I claim:
 1. Apparatus for coating semiconductor wafers comprising:afirst member having a horizontal track; support means contained on saidtrack for supporting a semiconductor wafer in a horizontal plane; meansfor rotating the first member about a fixed vertical axis at asufficient speed to disperse liquid on the surface of said wafer; andmeans for moving the support means and the wafer along said track duringthe rotation of the first member about the fixed axis, thereby to makethe liquid distribution on the wafer surface more uniform.
 2. Apparatusfor coating semiconductor wafers comprising:a first elongated member; asecond elongated member fixed at a first end thereof to a first end ofthe first elongated member; means for rotating the first and secondmembers about a fixed axis extending transversely to both the first andsecond elongated members and in proximity to the first ends of both thefirst and second elongated members; support means contained on the firstelongated member for supporting a semiconductor wafer, said wafer beingoriented to support a liquid on a surface thereof; a counterweightattached to the second elongated member; means for moving the supportmeans and the wafer along a portion of the length of the first memberduring its rotation, while keeping the fixed axis of rotationstationary, whereby the location of the axis of rotation of the waferrelative to the location of the wafer changes during the wafer rotation,thereby to make the liquid distribution on the wafer more uniform; andmeans for moving the counterweight along a major portion of the secondmember during its rotation such that the distance between the center ofthe wafer and the fixed axis of rotation, and the distance between thefixed axis of rotation and a center of the counterweight aresubstantially equal.
 3. The apparatus of claim 2 wherein:the first andsecond elongated members each contain central passages; first and secondpistons are contained in the passages of the respective first and secondelongated members; the support means is coupled to the first piston ofthe first elongated member; and the second piston constitutes at leastpart of the counterweight.
 4. The apparatus of claim 3 wherein:theweight of the counterweight is approximately equal to the combinedweights of the first piston, the support means and the wafer.
 5. Theapparatus of claim 4 wherein:the support means is constrained by a trackon the exterior of the first elongated member and is magneticallycoupled to the first piston.
 6. The apparatus of claim 4 wherein:thesupport means is constrained by a track on the exterior of the firstelongated member and is coupled to the first piston by a rigid U-shapedmember which is connected to the piston, extends out an open end of thefirst elongated member and is connected to the support means.
 7. Theapparatus of claim 4 wherein:the means for moving the support means andthe counterweight comprise means for applying air pressure to the firstand second pistons.